crypto: mxs - Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/crypto/mxs-dcp.c:103:1: warning:
 symbol 'global_mutex' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>